The specific detection of molecules is a keystone in understanding the role of that molecule in the cell. Labels, e.g., those that are covalently linked to a molecule of interest, permit the ready detection of that molecule in a complex mixture. The label may be one that is added by chemical synthesis in vitro or attached in vivo, e.g., via recombinant techniques. For instance, the attachment of fluorescent or other labels onto proteins has traditionally been accomplished by in vitro chemical modification after protein purification (Hermanson, 1996). For in vivo attachment of a label, green fluorescent protein (GFP) from the jellyfish Aequorea victoria can be genetically fused with many host proteins to produce fluorescent chimeras in situ (Tsien, 1998; Chalfie et al., 1998). However, while GFP-based indicators are currently employed in a variety of assays, e.g., measuring pH (Kneen et al., 1998; Llopis et al., 1998; Miesenböck et al., 1998), Ca2+ (Miyawaki et al., 1997; Rosomer et al., 1997), and membrane potential (Siegel et al., 1997), the fluorescence of intrinsically labeled proteins such as GFP is limited by the properties of protein structure, e.g., a limited range of fluorescent colors and relatively low intrinsic brightness (Cubitt et al., 1995; Ormö et al., 1996).
To address the deficiencies of GFP labeling in situ, Griffen et al. (1998) synthesized a tight-binding pair of molecular components: a small receptor domain composed of as few as six natural amino acids and a small (<700 dalton), synthetic ligand that could be linked to various spectroscopic probes or crosslinks. The receptor domain included four cysteines at the i, i+1, i+4, and i+5 positions of an α helix and the ligand was 4′,5′-bis(1,3,2-dithioarsolan-2-yl)fluorescein (FLASH). Griffen et al. disclose that the ligand had relatively few binding sites in nontransfected mammalian cells, was membrane-permeant and was nonfluorescent until it bound with high affinity and specificity to a tetracysteine domain in a recombinant protein, resulting in cells being fluorescently labeled (“FLASH” labeled) with a nanomolar or lower dissociation constant. However, with respect to background binding in cells, Stroffekova et al. (2001) disclose that FLASH-EDT2 binds non-specifically to endogenous cysteine-rich proteins. Furthermore, labeling proteins by FLASH is limited by the range of fluorophores that may be used.
Receptor-mediated targeting methods use genetically encoded targeting sequences to localize fluorophores to virtually any cellular site, provided that the targeted protein is able to fold properly. For example, Farinas et al. (1999) disclose that cDNA transfection was used to target a single-chain antibody (sFv) to a specified site in a cell. Farinas et al. disclose that conjugates of a hapten (4-ethoxymethylene-2-phenyl-2-oxazolin-5-one, phOx) and a fluorescent probe (e.g., BODIPY Fl, tetramethylrhodamine, and fluorescein) were bound with high affinity (about 5 nM) to the subcellular site for the sFv in living Chinese hamster ovary cells, indicating that the targeted antibody functioned as a high affinity receptor for the cell-permeable hapten-fluorophore conjugates. Nevertheless, functional sFv expression may be relatively poor in reducing environments.
Thus, what is needed is an improved method to label a desired molecule.